In the field of optical communication, differential phase-shift keying (DPSK) is a promising modulation scheme that provides high receiver sensitivity, high tolerance for main nonlinearities in high speed transmission and coherent crosstalk. The appearance of the interleaver technologies upgrades the design concept of wave separating/combining devices in dense wavelength division multiplexing (DWDM) and greatly reduces the design pressure of dense wavelength division multiplexor, while these devices are based on interferometers. The interferometer enables two light beams to transmit with the optical path difference (OPD) thereof being an expected value. However, the control of the interference cavity length difference is difficult. The change of the optical path difference directly affects the light interference intensity, thus the ambient temperature needs to be controlled so as to ensure the stability of the optical path difference. To balance the change due to the environment temperature variation, U.S. Pat. No. 6,924,894 discloses through disposing two heater and environment temperature sensors connected with a controller at the two interference arms, controlling the heaters and thereby balancing the influence of the environment temperature. This means must use complex electric circuits to eliminate the influence of the environment temperature. The cost is relatively high and the electric circuits are complex.